Introduction
1. Field of the Invention
The present invention relates to operating circuits for gaseous discharge lamps and, more particularly, to a circuit and method for operating a gaseous discharge lamp by controllably interrupting lamp current.
2. Description of the Prior Art
A low pressure mercury vapor discharge lamp, such as a fluorescent lamp, is an electrical device which exhibits certain special electrical characteristics, among them is a negative impedance characteristic which means that after the arc condition of the lamp has been struck or established, increased current through the discharge medium within the lamp results in decreased voltage between the electrodes of the lamp. Due to this characteristic of discharge lamp operation, it has been necessary to provide circuitry for current limitation in power supply circuits for operating discharge lamps. If current limitation is not provided, lamp failure or burnout of the power supply circuit generally results. Therefore, the prior art has typically provided electrical impedance elements connected in series with the fluorescent lamp for current control.
U.S. Pat. No. 3,771,013, issued Nov. 6, 1973 to Roche et al describes a lighting system which employs a static d-c power supply circuit to operate fluorescent lamps. The Roche et al patent describes a fluorescent lamp power supply circuit for operating a specially designed lamp on d-c power within the positive region of its volt-ampere characteristic. The source voltage applied to the lamp is reduced in the event that the lamp is operated outside of the positive region of its volt-ampere characteristic which is sensed when a predetermined maximum current has been reached, so that the current supplied to the lamp is monitored and maintained below a predetermined maximum current level at which runaway or damage of the lamp could occur. An analysis of fluorescent lamp runaway is presented in the paper titled, "Current Runaway in Fluorescent Lamps", which appeared in the Journal of IES, October 1972, by John F. Waymouth. The analysis in that paper concludes that in response to the application of d-c voltage V, see FIG. 2a of the present application, current takes an initial "instantaneous" step to i.sub.1 in about 2 microseconds and then increases exponentially with time as shown in FIG. 2b of the present invention. Given this analysis, it was considered necessary to clip the lamp current at a certain predetermined level to prevent current runaway and destruction of the lamp. The analysis in the paper of Waymouth states on page 43, right hand column, lines 13-16, that "the circuitry problem is that for any current greater than zero and voltages equal to V.sub.S, the starting voltage [emphasis in original] of the discharge, all V, and i points are in the domain of ##EQU1## hence current increases steadily with time until destruction," in which dne/dt represents the time rate of change of electron density. Waymouth controlled the lamp current level by cutting off the lamp power supply circuit before lamp current reached a predetermined level (Page 46, left column, lines 1-6).